1. Field of the Invention
The invention relates generally to an optical fiber coherent radiation source, and more specifically to an active multimode graded index fiber laser used to generate an optical signal in optical fiber communication systems, sensors and other optical fiber devices for technical and medical applications.
2. Prior Art Statement
Optical fiber lasers are becoming important components of optical fiber communication systems sensors, fiber-gyros and other optical fiber devices for different technical and medical applications. These lasers essentially consist of a rare-earth-doped optical fiber pumped with an external light source such as a single-mode semiconductor laser or diode-laser array. Mirrors can be used to provide the system with the necessary feedback.
Most fiber lasers are based on conventional high-purity silica-based optical fibers produced by chemical-vapor-deposition and are doped, for example, with such rare-earth elements as Nd.sup.3+, Er.sup.3+, Tm.sup.3+, Ho.sup.3+, Yb.sup.3+, Sm.sup.3+ and Pr.sup.3+ via gas-phase or solution-doping processes. Other types of fiber lasers include multicomponent high-gain glass fiber lasers fabricated by the rod-intube method or fiber lasers in fluoride based glasses. A single-mode optical fiber with rare-earth doped active core having a diameter between 3 and 8 um is typically used in fiber lasers because it can easily be integrated into single-mode fiber-communication and sensor networks. The pump light from high-power semiconductor sources (such as diode-laser arrays) couples into a highly multimode undoped cladding region which guides the pump radiation by virtue of its lower refractive index. Amplification of the light takes place only in a single-mode rare-earth-doped core while a portion of the mode field propagates in the passive cladding region. Different types of fiber laser cavity configurations may be used including a fiber directly coupled to dichroic mirrors (such as bulk optical elements or coatings deposited directly on the fiber end face) and an all-fiber laser cavity design employing fiber-loop mirrors.
Fiber lasers offer several advantages over traditional lasers. These advantages include the high pumping intensity available as a consequence of the good field confinement over a large length in the fiber. Additionally, because these lasers comprise a fiber, they have good compatibility with fiber-communication and sensor networks. The nature of rare-earth doping levels also allows output tunability over a broad range and a wide choice of pump wavelength-division multiplexing. Wide gain bandwidth makes rare-earth-doped fibers very attractive for use as optical amplifiers, especially for wavelength-division multiplexing applications. Moreover, spectral output of the fiber lasers is relatively insensitive to temperature changes compared to semiconductor-based sources and the small fiber cross section allows operation without special cooling.
The primary disadvantage of single-mode fiber lasers is the difficulty of coupling the pump light from diode laser arrays into the fiber. Even using a diffraction-limited semiconductor laser as the pump source fails to alleviate the problem due to the laser's low output power. The problem of coupling pump light into an active fiber can be solved, however, by using larger core diameters, i.e. multimode fiber lasers. Such multimode fiber lasers are much more powerful because of the more efficient pumping of the active core and more efficient utilization of the pump power. The mirror-coating-damage problem can be solved as well by using larger core diameters as found in multimode active fibers.